Generalize the logic for determining if we can make a BarBuilder out of a Bar.

If you have an `@AutoValue` class `Foo` with a property `Bar bar()` and `Foo.Builder` has a method `BarBuilder barBuilder()`, then we previously allowed that, provided we knew how to make a `BarBuilder` (for example by calling `Bar.builder()`). But if, in addition, `Foo` has its own `toBuilder()` method, then we also need to be able to make a `BarBuilder` out of an existing `Bar`, because if you call `Foo.Builder builder = foo1.toBuilder()` then the builder returned by `builder.barBuilder()` must start off with the contents of `foo1.bar()`. Previously we allowed that when `Bar` had a method `BarBuilder toBuilder()`. We also had a special case for Guava classes like `ImmutableList`. `ImmutableList` doesn't have a `toBuilder()` method, but you can make an `ImmutableList.Builder` out of an `ImmutableList` by starting with an empty `ImmutableList.Builder` and calling `addAll(theImmutableList)`. The change here generalizes that special case: it will work if the `BarBuilder` class has a method `addAll` or `putAll` that can accept an argument of type `Bar`. `ImmutableList.Builder<E>` has a method `addAll(Iterable<? extends E>)` and you can pass an `ImmutableList<E>` to that method, so we still accept `ImmutableList.Builder<T> barBuilder()`. But we now also accept any other class that qualifies.

Fixes https://github.com/google/auto/issues/794.

RELNOTES=Generalized the logic for determining if we can make a BarBuilder out of a Bar. For example, if your `@AutoValue` class `Foo` has a property `IntList ints()`, then your builder can have `IntListBuilder intsBuilder()`, where `IntListBuilder` is your own type, and this will work even if there is a `Foo.toBuilder()` method, provided it's possible to call `IntListBuilder.addAll(IntList)`. Previously it worked, but not if there was a `Foo.toBuilder()` because we didn't know how to make `IntListBuilder` out of `IntList`.

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